Frequency modulation of a carrier signal (FM) has been used extensively in both radio and wire transmission systems. Frequency modulation systems have the potential for high fidelity essentially noise free transmission of intelligence. However, achievement of this potential depends upon the fidelity of both the modulation and detection of the transmitted signal free from internal circuit noise and receipt of a reasonably strong signal at the receiver. For purpose of discussion of the present invention, the parameters of a standard commercial FM broadcast signal are employed. In the United States, FM broadcast carrier signals lie between 87.5 mHz and 108.0 mHz. The transmitted carrier is modulated plus and minus 75 kHz. The signal received at an FM receiver is mixed with the output of a tunable local oscillator to generate an Intermediate Frequency (IF) signal (e.g., 450 kHz modulated in accordance with the received radio frequency signal).
An FM detector is essentially a frequency to voltage converter. Early FM detectors comprise differentiators, diode detectors and filters. The output of a prior art FM detector is an "S" curve with zero output in the presence of an IF signal without modulation and plus and minus signals in the presence of modulation on the IF signal. Only the linear portion of the "S" curve is usable for FM reception. The coils and resistors of such prior art circuits do not translate to construction as integrated circuits.
With the advent of switched capacitor circuits to simulate resistors in integrated circuits, integrated circuit frequency to voltage converters have been suggested. An example of a frequency to voltage converter is found in an article entitled "Switched-Capacitor Frequency-to-Voltage and Voltage-to-Frequency Converters" published in the IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS VOL 33, NO 6, August 1986. The referenced article suggests the use of the circuits disclosed therein as being suitable for use in a modem for FM signals. However, there is a serious possibility of internally generated noise when the suggested circuits are employed as an FM detector in a receiver for a standard commercial broadcast signal. The unwanted noise at the output of the detector occurs as beat frequencies of the clock signals generated at the input from the received FM signal and the fixed rate clock signals employed in the switched capacitor feedback circuit.
It is desirable to have an integrated circuit FM detector which uses switched capacitor circuits that do not generate undesirable noise due to beat frequencies.